HEART-RATE AND STROKE VOLUME CONTRIBUTIONS TO CARDIAC-OUTPUT IN SWIMMING YELLOWFIN TUNA - RESPONSE TO EXERCISE AND TEMPERATURE

Cardiac performance in the yellowfin tuna (Thunnus albacares, 673-2470 g, 33-53cm fork length, FL) was examined in unanesthetized fish swimmi ng in a large water tunnel, Yellowfin tuna were fitted with either ele ctrocardiogram electrodes or a transcutaneous Doppler blood-flow probe over the ventral aorta and exposed to changes in swimming velocity (r ange 0.8-2.9FLs(-1)) or to an acute change in temperature (18-28 degre es C), Heart rates (fH) at 24+/-1 degrees C (30-130 beats min(-1)) wer e lower on average than previous measurements with non-swimming (restr ained) tunas and comparable with those for other active teleosts at si milar relative swimming velocities, Although highly variable among ind ividuals, fH increased with velocity (U, in FL s(-1)) in all fish (fH= 17.93U+49,93, r(2)=0.14, P<0,0001), Heart rate was rapidly and strongl y affected by temperature (Q(10)=2.37), Blood flow measurements reveal ed a mean increase in relative cardiac output of 13.6+/-3.0% with exer cise (mean velocities 1.23-2.10FLs(-1)) caused by an 18.8+/-5.4% incre ase in fH and a 3.9+/-2.3% decrease in stroke volume, These results in dicate that, unlike most other fishes, cardiac output in yellowfin tun a is regulated primarily through increases in fH. Acute reductions in ambient temperature at slow swimming velocities resulted in decreases in cardiac output (Q(10)=1,52) and fH (Q(10)=2.16), but increases in s troke volume (Q(10)=0,78), This observation suggests that the lack of an increase in stroke volume during exercise is not due to the tuna he art operating at maximal anatomical limits.